There is a conventional semiconductor light emitting device formed by a gallium-nitride based compound semiconductor for emitting bluish (ultraviolet to yellow) light has a structure, for example, as shown in FIG. 4. That is, a sapphire substrate 21 has a low-temperature GaN buffer layer 22 and a high-temperature n-type GaN (cladding) layer 23 epitaxially grown thereon. On the n-type layer 23 is formed an active (light emitting) layer 24 of a material having a bandgap energy lower than that of the cladding layer so as to determine an emission-light wavelength, e.g. a compound semiconductor based on InGaN (having a variable ratio of In and Ga). A p-type (cladding) layer 25, having a compound semiconductor sublayer 25a and a GaN sublayer 25b, is formed on the active layer 24. A p-side electrode 28 is formed on a surface of the p-type layer 25, while an n-type electrode 29 is formed on the n-type layer 23 exposed by partly etching away the overlying semiconductor layers. Incidentally, the n-type layer may include an AlGaN-based semiconductor sublayer on an active layer 23 side in order to enhance carrier confining effects.
As stated above, the conventional bluish-light semiconductor light emitting device employing a gallium-nitride based compound semiconductor is formed by the layers overlaid on the sapphire substrate, which layers include those of a GaN-based compound semiconductor, an AlGaN-based compound semiconductor, InGaN-based compound semiconductor, and so on. However, sapphire has a lattice constant of 4.76 angstroms, and GaN has a lattice constant of 3.18 angstroms. Al.sub.0.1 Ga.sub.0.9 N, for example, has a lattice constant of 3.12 angstrom, and In.sub.0.05 Ga.sub.0.95 N has 3.198 angstroms. Thus, these layers are different in lattice constant. These semiconductor layers, when formed overlying one another, tend to induce distortions of crystal lattice. This raises such problems that electric current is difficult to flow through the layers, and cracks are induced in the crystal reaching to the light emitting (active) layer, resulting in lower in light emitting efficiency. In particular, the n-type layer is formed thick, i.e. approximately 2-5 .mu.m, on the sapphire substrate with a largely-different lattice constant through a thin low-temperature buffer layer. Therefore, the n-type layer tends to cumulate crystal distortions and ready to induce lattice mismatch therein.